Continuing studies of hormonal mechanisms in obsessive-compulsive disorder (OCD) found abnormalities in cortisol secretion in a subgroup of OCD patients with episodic symptoms, in contrast to normal cortisol secretion in the larger group of OCD patients with enduring symptoms, suggesting involvement of endocrine mechanisms in a patient subgroup. Studies of the role of the neuropeptide oxytocin in regulating stress and anxiety systems possibly involved in OCD continued in the past year with a study of the effects of lactation on the response to psychological stress, and by a longitudinal study of the natural history of OCD symptoms during pregnancy and lactation. A study investigating the involvement of specific brain regions in OCD symptoms has used the new technique of repetitive transcranial magnetic stimulation (rTMS). rTMS permits the direct stimulation of the cerebral cortex, making it a potentially useful probe of brain function in OCD. We found that stimulation over the right prefrontal cortex temporarily reduced compulsive urges, consistent with evidence implicating prefrontal cortex regions in OCD symptoms. Our section has continued long-standing efforts to relate the symptoms of OCD to the functioning of brain serotonin systems. A study found that a serotonin receptor blocker increased symptoms in OCD patients who had improved on the antiobsessional medicine fluoxetine, but not in unmedicated patients. Three functional neuroimaging studies of the effects of different serotonergic agents on OCD symptoms and brain blood flow are complete or nearly complete. A genetic study found that the gene coding for the serotonin transporter, the site of action of anti-OCD medications, was itself normal in a group of OCD patients. However, a gene which may affect how much of the serotonin transporter is actually produced has been identified. We have begun a study to determine if genetic variability at this site is associated with OCD.